Colliding gravitational plane waves with noncollinear polarization. II

A simple criterion for colliding gravitational plane waves is developed. This colliding wave condition is preserved by a new realization of the Geroch group augmented by a Kramer--Neugebauer involution. A three-parameter generalization of a two-parameter family of solutions with noncollinear polarization discovered recently by Ferrari, Ibanez, and Bruni is presented, and two additional solutions are derived that demonstrate that much larger families are likely to be constructed in the near future